The determination of total and partial gas pressures in water and other liquids provides valuable information as to the degree to which equilibrium with the gaseous environment or the atmosphere has been established, and the differential gas pressure between the solution and its environment. For the purposes of this discussion and description, total dissolved gas pressure in a liquid means the sum total of all partial pressures of gases dissolved in the liquid including the vapor pressure of the liquid. In other words it is a direct evaluation of Dalton's Law for a mixture of gases but performed in a liquid. This parameter is of importance in many areas. Studies to determine relationships between excess pressures and environmental conditions which have created supersaturation problems require the measurement of total dissolved gas pressure. It is an indication of saturation state which has importance in hydrology, fisheries, aquaculture and industry.
Fish and aquatic life in rivers, lakes, hatcheries, aquaria and other aquaculture projects have often died either for lack of oxygen from deficient saturation or from gas embolism because of the excess total pressure of dissolved gases in these various bodies of water. Such conditions facilitate bubble formation in the organisms, similar to air embolism in divers, and often with fatal results. As a consequence, instruments capable of quickly and easily providing and automatically monitoring the dissolved gas pressure information are currently used and increasingly needed to monitor waters where there is any likelihood of danger or risk to fish and aquatic life.
As those skilled in the art are aware, water in which there is as little as 10 percent excess of dissolved gas can be stressful or lethal to fish. Any pumped or otherwise pressurized water supply can present a risk and hence it is necessary to know the levels of air dissolved gases in a particular system. In addition, many industries aerate or sparge water or other fluids with air or other gases to saturate with or remove other gases. Measuring techniques and new sensors such as that herein described will facilitate economical quality control where used.
At the present time no industrial grade sensors exist suitable for this purpose with the exception of those mentioned above which were primarily designed for fisheries and aquaculture and which have several accompanying deficiencies.
Individual instruments and techniques for measuring dissolved gas and fluid vapor pressures in solutions have for the most part been concerned with particular gaseous components. Some of the more obvious applications of a device for measuring total dissolved gas pressure are in the area of water pollution, industrial and other waste water analysis, fish hatchery water quality, aquarium water quality, and wine, beer and beverage production. There are other applications where it is desired to assess the state of gas pressure equilibrium or disequilibrium between the water or fluid and a gas phase. Accordingly, the invention's application to water quality is an obvious example of general applications requiring knowledge of the saturation state of any liquid with respect to ambient hydrostatic pressure and with respect to atmospheric pressure. Clearly, these more general uses include numerous industrial, research and even space applications and provide a new analytical method of greater simplicity and convenience.
Current state of the art instrumentation is unnecessarily cumbersome and expensive. Some of the prior non-electronic instruments, sometimes referred to as "saturometers" or "gasometers" require time consuming and tedious procedures, sometime require water pumps, and as a result present prohibitive disadvantages if a large number of measurements must be taken to monitor a relatively large body of water, or if remote measurements or measurements at depth must be made. Additionally, known instruments and their use in the field require skill in and training for the operators, are susceptible to membrane damage and are time consuming to repair.
Also such instruments do not provide an absolute pressure reading but only a gauge or differential pressure which due to barometric pressure fluctuations prevents calibration of percent saturation and is subject to physical constraints. Furthermore, the use of dial gauges employing a Bourdon tube with a considerable internal volume imposes further equilibration time requirements and gradual gauge errors due to corrosion. Further, the alternative of using mercury in an open-ended manometer while having the advantage of providing a true differential reading increases the size of the devices using it, involves positional constraints and always involves environmental and health hazards if spilled. Such instruments also will require an operator or observer at the measuring site which increases the cost of measurements and eliminates the utility of the devices in automatic process control.
The existing instrumentation for performing the measurement of total dissolved gas pressure, including the devices described in the patents infra, have the disadvantage of requiring knowledge and experience of a specialist in making the required measurements. All the previous devices require tedious disassembly for replacement of the sensing membrane if it is punctured or otherwise damaged or blocked. Such prior art devices are limited in this respect by the large amount of silicon rubber tubing needed to overcome their internal volume and the slow response time. Commercially available models are also limited by the amount of silicon rubber tubing which can be interfaced with the pressure transducer and still allow both ease of changing the membrane, and a reasonable equilibration time. The instant invention overcomes these disadvantages by being more amenable to rapid total replacement or rapid membrane repair.
Current state of the art instrumentation is shown in U.S. Pat. No. 3,871,228; U.S. Pat. No. 4,366,700; U.S. Pat. No. 4,563,892; and U.S. Pat. No. 4,662,210. The last patent listed measures multiple parameters but its utility is limited because of the size of the sensor probe, relatively slow speed of response, replacement expense, eventual condensate formation inside the tubing and manufacturing and maintenance costs. Additionally in U.S. Pat. No. 4,662,210 the apparatus requires separate configuration of the membrane tubing separating the water from the gas phase and connection of this by means of a type of narrow tubing placed through a waterproof housing to the pressure sensing device. The waterproof housing equipped with a feed through to connect to the housing contains the pressure sensor which must be chosen for minimal internal volume. Choices of pressure sensors with low internal volume are necessarily limited and those that are available are often of a shape or configuration which does not facilitate compact or convenient design of the resulting probe. Also many of the most sensitive pressure sensors involve a large surface area which increases internal volume when interfaced with such tubing. In particular, the size of the probe limits its application where small size is necessary. Also, replacement costs are high and some labor of specialized personnel is required. A further shortcoming of existing methods including the listed patented systems is the potential for formation of liquid condensate inside the lumen of the dimethyl silicon tubing. The condensation can reduce the accuracy of the pressure reading as cross bridging of pure liquid drops inside the capillary causes meniscal forces to affect the total gas reading.
Thus, it becomes apparent that the preferred approach to overcoming the difficulties discussed above is an integrated design as set forth and claimed hereinafter.